The present invention relates generally to the manufacture of creped products such as, e.g., bath tissue, paper towels, napkins, etc. More particularly, the present invention relates to systems and methods for predicting natural coating transfer in real time via continuous online data monitoring, and enabling real time control of the manufacturing process based thereon.
Conventional processes for the manufacture of creped products such as bath tissue, paper towels and napkins are well-established and require little elaboration herein. Generally stated, a continuous wet fibrous sheet is generated from a pulp stock having characteristics defined in part by the particular combination of one or more constituent fiber sources, and further in view of chemical additives, water source and the like. A heated rotary drying cylinder (herein referred to as a “Yankee dryer”) is configured to pick up the wet sheet, to substantially dry the sheet, and then crepe the sheet in combination with a creping doctor blade associated therewith. This creping process imparts a three-dimensional structure to the sheet that is responsible, e.g., for the soft feel of tissue products. Creped products can be made using (but not limited to) light dry crepe machines, wet crepe machines, as well as through air drying (TAD) and other machines that may impart a structure to the sheet prior to the Yankee dryer.
The creping process, and more particularly the surface conditions on the Yankee dryer, are critical factors in the overall manufacturing process. For the sheet to attach to the Yankee surface there must be a thin adhesive coating present. This adhesive coating will in fact aid in the pickup of the sheet. The strength of the adhesive force between the Yankee surface and the sheet is very important factor in tissue manufacture. The force must be strong enough to hold the sheet in place, but weak enough to release the sheet at the proper point. Specifically designed chemical formulations are applied to the Yankee surface to provide the necessary adhesion and release properties of the surface. The pulp stock that provides the material that forms the web fibrous sheet also includes substances that will stick to the Yankee surface and provide an adhesive force. In this industry the term “natural coating” is used for this material that naturally comes from the stock and coats the surface of the Yankee. The composition of the pulp stock changes as the fiber sources or additives in that stock change, or as the characteristics of the water change. This variation requires adjustment in the amount of the chemical formulations that are used to control the adhesion and release properties of the Yankee surface. The “natural coating” plus the chemical additive together provide the total adhesive force.
Conventional techniques for adjusting the adhesive coating feed rate to achieve proper characteristics on the Yankee dryer are labor- and time-intensive, and further rely on assumptions regarding machine operation. As one example of a known process flow, the user is prompted to adjust the coating feed rate based on a fiber source (furnish) change, such as for example in view of a change in tissue grade. A mill employee or chemical supplier sales representative may, perhaps within minutes of the furnish change, obtain and begin testing of a sample to determine characteristics such as the total suspended solids (TSS) therein. This process is not online and therefore is not instantaneous or otherwise conducted in real time. The user can then inspect the set points for stock flow and machine speed, via for example a machine control system, for the given creped product grade and calculate the natural coating potential using a predetermined equation. However, this requires the assumption that the machine is operating at the stated set points.
Understanding and monitoring the amount of natural coating is an important part of improving Yankee adhesive performance which leads to better production of creped products. It would therefore be desirable to measure relevant online process characteristics and subsequently predict the amount of natural coating available to transfer to the coating, substantially in real time or at any given selected time. However, the inherently dynamic nature of the creped product manufacturing process has traditionally made such predictive analysis and corrections extremely difficult and impractical.